ALKYLATION REACTIONS OF DIALKYLZINC COMPOUNDS WITH 1,4-DIAZA-1,3-BUTADIENES - CATIONIC AND RADICAL ANIONIC ORGANOZINC INTERMEDIATES - MOLECULAR-STRUCTURE OF THE CATIONIC ORGANOZINC SPECIES [MEZN(T-BUN=CHCH=N-T-BU)]O3SCF3 AND ME2ZN(BPY) (BPY=2,2'-BIPYRIDINE)

1,4-Disubstituted 1,4-diaza-1,3-butadienes, R'N=CHCH=NR'(R'DAB), are r egioselectively alkylated by R2Zn compounds. It is suggested that the mechanism of these alkylation reactions involves prior formation of th e 1:1 R2Zn(R'DAB) complex (R = alkyl) followed by intramolecular ligan d-to-ligand electron transfer, generating, via homolytic R-Zn bond cle avage, a [RZn-(R'DAB)]./R. radical pair. Collapse of this latter pair results in the formation of N- or C-alkylation products. In the presen t study one aspect of a possible polar mechanism is probed, for which organozinc radical anions [R2Zn(R'DAB)].- and organozinc cations [RZn( R'DAB)]+ have been suggested as the key intermediates. Representative examples of such organozinc cations, i.e. the title compound [MeZn(t-B uDAB)]OTf (9) and [(2,6-xylyl)Zn(t-BuDAB)]OTf (10), were separately pr epared from their corresponding 1:1 coordination complexes Me2Zn(t-BuD AB) and (2,6-xylyl)2Zn(t-BuDAB), respectively, by their reaction with 1 equiv of trifluoromethane-sulfonic acid. The X-ray crystal structure of 9 shows a four-coordinate zinc atom with a distorted-tetrahedral g eometry, wherein the triflate is bonded almost perpendicular to the Me Zn(t-BuDAB) plane. Structural parameters in the MeZn(t-BuDAB) plane ar e in good agreement with ab initio results for the MeZn(H-DAB) cation. The X-ray crystal structure determination of Me2-Zn(bpy) (1c; bpy = 2 ,2'-bipyridine) has been carried out to compare the structural feature s of two neutral complexes, e.g. Me2Zn(bpy) and Me2Zn(t-BuDAB), with c ationic species such as 9. F-19 NMR spectra of 9 and 10 in benzene and tetrahydrofuran indicate that in solution the triflate-zinc bond diss ociates to give the solvated cationic species [RZn(t-BuDAB)]+ (R = Me, 2,6-xylyl). The title compound 9 reacts with 1-(trimethylsiloxy)-1-et hoxycyclopropane via C-alkylation of the NCCN skeleton to give a 2-pyr rolidinone derivative (11). Crystal data: 1c, C12H14N2Zn, orthorhombic , Pbam (No. 55), a = 12.0547(8) angstrom, b = 6.4374(4) angstrom, c = 7.8186(3) angstrom, V = 606.73 angstrom3, Z = 2, R = 0.046; 9, C12H23N 203F3SZn, monoclinic, P2(1)/a (No. 14), a = 12.345(2) angstrom, b = 11 .363(2) angstrom, c = 13.260(3) angstrom, beta = 108.33(2)-degrees, V = 1765.7(6) angstrom3, Z = 4, R = 0.068.